The present invention relates to physiological monitoring equipment and, in particular, monitoring equipment that include processes for quantitatively estimating the quality of the detected signals corresponding to physiological measurements and which provide appropriate feedback to the clinician based on that estimate of signal quality.
Typically, for physiological monitoring instruments, the instrument is unable to accurately determine the accuracy and reliability, or a quality of a signal obtained from the sensor. An example of such physiological monitoring instrument is a pulse oximeter. Pulse oximetry is typically used to measure various blood characteristics including the blood oxygen saturation of hemoglobin in arterial blood and the pulse rate of the patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor that passes light through a portion of a patient's blood perfused tissue and photo-electrically senses the absorption and scattering of light in such tissue. The amount of light absorbed and scattered is then used to estimate the amount of blood constituent in the tissue using various algorithms known in the art. The “pulse” in pulse oximetry comes from the time varying amount of arterial blood in the tissue during a cardiac cycle. The signal processed from the sensed optical measurement is the familiar plethysmographic waveform due to cyclic light attenuation.
The accuracy of the estimates of the blood flow characteristics depends on a number of factors. For example, the light absorption characteristics typically vary from patient to patient depending on their physiology. Moreover, the absorption characteristics vary depending on the location (e.g., the foot, finger, ear, and so on) where the sensor is applied, and whether there are objects interfering between the sensor and the tissue location (e.g., hair, nail polish, etc.). Further, the light absorption characteristics vary depending on the design or model of the sensor. Also, the light absorption characteristics of any single sensor design vary from sensor to sensor (e.g., due to different characteristics of the light sources or photo-detector, or both). The clinician applying the sensor correctly or incorrectly may also have a large impact in the results, for example, by loosely or firmly applying the sensor or by applying the sensor to a body part which is inappropriate for the particular sensor design being used.
The clinician needs to know how accurate or reliable of a reading is being provided by the instrument. Moreover, the instrument should ideally provide measurements that are accurate and reliable. A measure of the reliability and accuracy of a physiological measurement can be the quality of the signal. Although quality is a rather nebulous term, generally speaking, high quality signals are indicative of more reliable and more accurate physiological measurements and conversely, low quality signals are indicative of less reliable and accurate signals.
Some oximeters devices qualify measurements before displaying them on the monitor, by comparing the measured signals to various phenomenologically-derived criteria. These oximeters qualify the signal by making an assessment of its accuracy and only display values of estimated parameters when the signal quality meets certain criteria. The disadvantage of such approaches is that no feedback is provided to the clinician regarding what the clinician should do to improve the measurements. Worse yet, if the signal quality is deemed poor, and a decision is made by the device to not display the measured value, then the clinician is left with no more information than if the device was never used in the first place, which can be frustrating. Most typically, the clinician will end up removing the sensor from a particular tissue location to re-attach it to another location and heuristically repeats this process until more reliable measurements, deemed worthy of being displayed are provided by the instrument. The time duration where a measured value is displayed is known as the posting time. An ideal oximeter provides both accurate measurements and high posting times. While some instruments make estimates of signal quality, there still exists a need for improvements in this area; improvements that assess the quality of the signal while maintaining or increasing the posting times of the measured values. There is therefore a need for an improved and more quantitative assessment of signal quality. There is also a need for the monitoring equipment to guide the clinician to make necessary adjustments to improve the signal quality.
The issues related to obtaining high quality signals which are indicative of reliable and accurate measurements of critical physiological parameters, are further compounded when the sensor is “blindly” placed. A blindly placed sensor is one that is placed adjacent to a tissue location that is not readily observable by the clinician. An example of such a blind sensor placement is when a clinician uses a fetal pulse oximeter. The fetal pulse oximetry sensor is placed through the cervix during labor. The sensor, which is supposed to lie between the cheek of the fetus and the wall of the uterus, measures the fetus' blood oxygen levels. The sensor is connected to a monitor that displays the fetus' current blood oxygen levels. During the course of monitoring the quality of the signal obtained can vary significantly and rapidly. This change in the quality of the signal can be caused by sensor-to-fetus positioning irrespective of actual fetus physiological condition, resulting in spurious readings and interrupted posting times over relatively short time frames. Monitoring the heart rate of the fetus is an indirect way of assessing the amount of oxygen in the fetus' blood, which allows a clinician to determine whether the fetus is in distress during labor. Knowing the fetus' blood oxygen level during labor, and hence whether the fetus is in distress or not, allows a clinician to better decide as to whether to perform a cesarean versus letting the labor progress. Therefore, with a blindly placed sensor, and especially one used in a fetal pulse oximetry, the need for an improved assessment of signal quality is heightened. Moreover, just as in non-blindly placed sensors, there is also a need for the medical device to provide guidance to a clinician to improve the quality of the signal when the signal quality is so low as to indicate an inaccurate and/or unreliable estimate of a measured value.